They ocurr whether logged in or not and are usually assosciated with boat sales and insurance.
They tend to obscure the RHS of a post which means you don't get the full gist and is a right scunner.
There was an interesting thread recently about adverts etc. and Lakesailor recommended an addon for Firefox called Adblock plus. This has worked a treat for me.
The thread is here: http://www.ybw.com/forums/showflat.php?Number=2299316
I know these have been a pain for some people in the past but I never have been troubled by them. In fact they only appear at all on the other YBW.com pages. Every so often one gets out of hand and obscures those pages but not as far as i am concerned the forum pages. Dont see them at all on here.
BTW using IE8 but previously IE7 and earlier versions before that
Never seen them, Firefox 3, adblock and a very program called ad muncher.
It also helps if you block macromedia flash, available with Spyware Blaster (Free).
You don't say if you have the Firefox add-on AdBlock Plus. That sees them all of for me.
For even better performance, in Addblock preferences (under Tools in Firefox), go to filter >subscriptions and subscribe to the top one, Easylist. This adds all the regular offenders.
Using IE 8 and Ad Muncher I get no adds, however if I dissable Ad Muncher and refresh I then get an annoying moving Boats for Sale banner at the head of the page. Nothing down the side.